1. Field of the Invention
The invention relates to a time base changer suitable for use in a video converter for converting between a sequence of input video images at an input video rate and a sequence of output video images at an output video rate and to a video converter incorporating such a time base changer.
2. Description Of the Prior Art
Video converters are well known devices used to convert video signals from one video standard to another, for example, from an input interlaced high definition video signal (HDVS) having 1125 lines per frame and 60 fields per second into an output interlaced video signal having 625 lines per frame and 50 fields per second.
FIG. 1 is a schematic block diagram of a previously proposed video standards converter for the down conversion from high definition video signals (HDVS) in accordance with a standard of, say, 1125 lines, 60 Hz, and 2:1 interlace to normal definition video signals in accordance with a standard of, say, 625 lines, 50 Hz rand 2:1 interlace. The previously proposed video standards converter provides compensation for the effects of the movement of features between input video fields. The video standards converter 10 comprises a down converter 16 for receiving the high definition input signals 12. The input video signals are converted to a progressive format (i.e. the 2:1 interlace is removed) in accordance with a motion adaptive interpolation scheme in which interframe interpolation is employed in static picture areas and interfield interpolation is employed on moving material within the picture. This 1125 line, 60 Hz, 1:1 progressive format is then converted to 625 line, 60 Hz, 1:1 format which is then output via path 18 to a time base changer 20.
The time base changer 20 has the function of converting from the input frames at an input rate of 60 Hz to output frames at an output rate of 50 Hz. The output from the time base changer on lines 22 comprises two streams of video images, A and B, each at 50 Hz and a stream of interpolation coefficients, I, also at 50 Hz. It will be appreciated that less frames are needed at 50 Hz than at 60 Hz. Whereas the input to the time base changer has 60 frames per second, the output on each of the channels A and B from the time base changer has only 50 frames per second. Accordingly, it can be seen that the output video rate on each of the channels from the time base changer requires only 5/6 of the number of frames as at the input video rate.
The primary function of the time base changer 20 is to compute which input frames are required from the sequence of input video frames in order to generate the output sequence of video frames. The time base changer selects one of the input video frames to be output on each of the respective channels A and B for each output video frame. Each pair of frames on the channels A and B is passed to an interpolator 24 which responds to an interpolation coefficient generated by the time base changer. Each interpolation coefficient defines interpolation factors for combining the pair of frames output on the channels A and B in order to generate a corresponding output video frame.
The process of generating an output frame has to take account of the possible movement of objects within the image. If a feature moves from a first to a second image position on two successive frames of the input video, simple interpolation of those two frames will result in blurring that feature. In order to take account of this, a vector processor 26 is provided. The vector processor 26 calculates a position which is temporally (e.g. half way) between the position of the feature in two successive input frames. Each of these frames is then offset by half the amount of motion estimated between the two frames to align the features of the two images prior to interpolation. Further details of vector processing for motion compensation in a video standards converter can be found in GB-A-2 213 749.
The existing time base converter is designed for a fixed conversion between input video provided at a first frequency for generating output at a second frequency (i.e. from 60 to 50 Hz ). If, however, the input video is derived from a 30 Hz progressive scan input, e.g. from a computer graphics source, the conversion from 30 Hz to 50 Hz requires a time base changer to select different pairings of the input video frames and different interpolation factors for combining those input video frames. In the prior art a completely separate time base changer was necessary to deal with input video signals produced at different video rates.